Since the transistor was invented in 1948, tremendous advancements have been achieved in the progress of solid state device technology. These advancements have been made both through the development of more and more advanced device concepts, as well as through advancements in the materials which are used to fabricate the devices. By way of example, the performance achieved by integrated circuits today is the result of, among many other things, a considerable breakthrough in the 1950's relating to methods of growing pure and single-crystal silicon.
Conventionally, single crystalline silicon substrates have been widely used in semiconductor device manufacturing processes. After the formation of the single-crystal ingot by liquid-encapsulated Czochralski growth, the ingot is mechanically processed to manufacture silicon wafers.
Today, many semiconductor devices are highly integrated. Devices having high levels of integration may achieve high speed and performance levels, and may be more economically efficient to manufacture. However, various problems can also occur as the level of integration increases and the size of individual devices decreases. For example, as the channel length of a conventional planar field effect transistor (“FET”) is reduced, several potentially undesirable effects may occur including (1) a short channel effect such as a punch-through, (2) an increase in the parasitic junction capacitance between the junction region and the substrate, and (3) an increase in the leakage current of the transistor.
In efforts to reduce and/or to eliminate one or more of the above-mentioned problems, studies have been performed in which the semiconductor devices are fabricated on various different types of substrates. By way of example, many solid state devices have been formed using silicon-on-insulator or “SOI” substrates in which a silicon layer is formed on an insulating layer. The use of SOI substrates can improve the characteristics of a semiconductor device in several ways, including reduced junction leakage current, reduced short channel effect, a lower operation voltage, and increased isolation. However, the use of SOI substrates can also give rise to several disadvantages. For example, a “floating body” effect can occur in devices with SOI substrates as a result of heat generated during operation of the device or via an accumulation of hot carriers having high energy. SOI devices also may tend to have reduced reliability if the threshold voltage is changed, because a back bias is not applied in SOI devices. In addition, since SOI field effect transistor technology requires connecting two substrates, the process for fabricating SOI devices tends to be more complicated and costly as compared to device fabrication processes using conventional bulk silicon substrates.
One specialized category of semiconductor devices are micro-electromechanical systems (MEMS). MEMS technology may be used to create electric devices and machine parts which can be less than a millimeter in size. Typically, MEMS devices involve the fabrication of both electrical structures and mechanical structures on a micro scale using conventional semiconductor manufacturing techniques. A MEMS device might include, for example, various mechanical elements, actuators and electronics on a single silicon wafer.
Typically, the electronic components of a MEMS device are formed using conventional integrated circuit fabrication technology (e.g., CMOS, bipolar, BICOM processes), while the mechanical components are formed through a micromachining process that, for example is used to selectively etch part of the silicon wafer and/or to form mechanical or electromechanical elements on the substrate. With MEMS technology, all of the electrical and mechanical structures may be formed on a single silicon wafer.